The present invention generally relates to inkjet printing and, more specifically, to methods and structures for securing a printhead to an inkjet print cartridge, whereby ink is prevented from shorting printhead conductors together.
Substantial developments have been made in the field of electronic printing technology. Specifically, a wide variety of highly efficient printing systems currently exist which are capable of dispensing ink in a rapid and accurate manner. One such system is a thermal inkjet printer that utilizes ink cartridges. Thermal inkjet print cartridges operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of a plurality of orifices so as to print a dot of ink on a recording medium such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays in a nozzle plate. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved across the paper. The paper is typically shifted each time the printhead has moved across the paper. The thermal inkjet printer is fast and quiet, as only the ink strikes the paper. These printers produce high quality printing and can be made both compact and affordable.
A typical inkjet print cartridge is shown in FIG. 1 as print cartridge 20, which includes an ink reservoir 22 for containing liquid ink. The liquid ink is delivered to a printhead 24 that is formed of a flexible polymer tape 30 using Tape Automated Bonding (TAB). Printhead 24 includes a nozzle member 28 comprising offset holes or orifices 26 formed in the flexible polymer tape 30.
Referring to FIG. 2, a back surface of tape 30 includes conductors 42 formed thereon by, for example, using a conventional photolithographic etching and/or plating process. These conductors are terminated by large contact pads 32 (FIG. 1) designed to interconnect with a printer. Print cartridge 20 is designed to be installed in a printer so that contact pads 32, on the front surface of tape 30, contact printer electrodes providing externally generated energy signals to the printhead. Windows 34 and 36 (FIG. 1) extend through tape 30 and are used to facilitate bonding of the ends of the conductors to electrodes on a silicon substrate containing heater resistors.
In the standard print cartridge assembly, the conductors are formed on the back surface of tape 30 (opposite the surface which faces the recording medium). To access these conductors from the front surface of tape 30, holes (vias) must be formed through the front surface of tape 30 to expose the ends of the conductors. The exposed ends of the conductors are then plated with, for example, gold to form contact pads 32 shown on the front surface of tape 30. When print cartridge 20 is properly positioned in an inkjet printer, contact pads 32 are pressed against associated contacts on the inkjet printer so as to electrically couple the resistors to a source of electrical current.
Affixed to the back of tape 30 is a silicon substrate 38 containing a plurality of individually energizable thin film resistors. Each resistor is located generally behind a single orifice 26 (FIG. 1) and acts as an ohmic heater when selectively energized by one or more pulses applied sequentially or simultaneously to one or more of contact pads 32 (FIG. 1). Conductors 42 lead from the contact pads 32 to electrodes on substrate 38. Hole 40 allows ink from an ink reservoir to flow to the front surface of substrate 38.
The print cartridge structure has a number of drawbacks. For example, conductors that extend out from a flexible circuit and connect to electrodes on a substrate require adequate insulation on the bottom surface of the conductors formed on the bottom surface of the flexible circuit. During the course of printing, cleaning operations need to be done to prevent nozzles from clogging. In addition, spray from the ink ejection is generated. As a result, the ink manages to reach the underside of the flexible circuit, which causes some degree of shorting between conductors. Even at low voltage levels and at fairly low operating speeds, this shorting together of conductors can affect the operation of the printhead. Where higher performance printers and printheads are utilized, which require faster and faster speeds and possibly incorporate active demultiplexing circuitry on the printhead itself, the problem is compounded. Thus, high current power supply voltages and low current control signals that are or will be carried by conductors connected to currently available or future printheads, may result in ink shorting between these conductors which can significantly affect the characteristics of the control signals and may, therefore, cause significant fluctuations in print quality.
Previous solutions to the ink shorts problem have primarily focused on (1) modifying the design on top of the substrate, the layout and geometry of the thin film, thick film and the TAB bond window opening and (2) improving the chemical and mechanical robustness of the adhesive materials and interfaces.
U.S. Pat. No. 5,442,384, entitled xe2x80x9cIntegrated Nozzle Member and TAB Circuit for Inkjet Printhead,xe2x80x9d describes a novel nozzle member for an inkjet print cartridge having a barrier layer, as a separate layer or formed in the nozzle member itself, and including vaporization chambers surrounding each orifice and ink flow channels which provide fluid communication between an ink reservoir and the vaporization chambers. U.S. Pat. No. 5,648,805, entitled xe2x80x9cAdhesive Seal for an Inkjet Printhead,xe2x80x9d describes a procedure for sealing an integrated nozzle and flexible or tape circuit to a print cartridge, whereby a flexible circuit is adhesively sealed with respect to the print cartridge body by forming an ink seal, circumscribing the substrate, between the back surface of the flexible circuit and the body, thus providing a seal directly between a flexible circuit and an ink reservoir body. U.S. Pat. No. 5,736,998, entitled xe2x80x9cInkjet Cartridge Design for Facilitating the Adhesive Sealing of a Printhead to an Ink Reservoir,xe2x80x9d and U.S. Pat. No. 5,852,460, entitled xe2x80x9cInkjet Print Cartridge Design to Decrease Deformation of the Printhead When Adhesively Sealing The Printhead to the Print Cartridge,xe2x80x9d describe improved headland designs. However, these designs did not address the problem of ink shorts caused by ink leaking into the conductive leads and conductive traces of the flexible circuit.
On most flexible circuits, these leads are also protected on the back side by a laminated cover layer. For example, U.S. Pat. No. 5,442,386 describes a typical print cartridge assembly including a coating that is laminated to the back side of a tape on which the conductors are formed. The coating comprises a middle layer of polyethylene terephthalate (PET) and two additional outer layers composed of a copolyester film. The three layers are laminated together and provided on a roll. While this coverlayer resolves some of the shortcomings of the prior art methods and print cartridge assemblies, it requires use of a fully-cured and/or multiple-layered coverlayer which increases the cost of production dramatically and complicates the manufacturing process.
Other known print cartridge assemblies have simply relied on use of a single coating, usually consisting of a PET core, coated with a first layer of adhesive on one side and a second layer of adhesive on the other side of the PET core. However, there are a number of disadvantages to this current approach. Current coverlayer adhesives used in the industry are either soluble in ink and polyethylene glycol (PEG) (the latter being used in the printer service station), or will not adhere to adjacent materials used in the cartridge. In view of the solubility of these adhesives to common components in printers, the coverlayers containing such adhesives provide minimal protection to the flexible circuits located on the backside of the print cartridge.
While considerable gains have been made in both of these areas, they are limited in their effectiveness and an additional robustness margin is desired. Accordingly, there is a need for an improved method of encapsulating the flexible circuit leads that reduces ink shorts and corrosion due to ink penetration into the flexible circuit leads. More specifically, there is a need for a coverlayer material that is robust and stable in the presence of ink and PEG, which is able to maintain adhesion to the pen body, and which further prevents electrical shorts between traces on the circuit due to water vapor or ink.
The present invention includes an improved structure for an inkjet print cartridge and a method for securing a printhead to the cartridge, whereby ink is prevented from shorting the printhead conductors together.
In one embodiment of the invention, conductors formed on a surface of a flexible circuit are encapsulated by a layer of insulation to prevent ink seeping under the flexible circuit from shorting the conductors together. More specifically, a structure for preventing ink shorting of conductors connected to a printhead is provided. The structure includes a layer of insulating material shaped to at least partially encapsulate the conductors on the printhead. The insulating material includes a first surface and a second surface, with each of the surfaces having an adhesive coated thereon. The adhesive includes a polymer of ethylene and glycidyl methacrylate. In yet another embodiment of the invention, the adhesive further includes an acrylic ester. Alternatively, a first adhesive may be coated onto a first surface of the insulating material or coverlayer and a second adhesive (distinct in composition from the first adhesive) may be coated onto a second surface of the insulating material or coverlayer.
In another embodiment of the invention, a print cartridge for an inkjet printer is provided. The print cartridge includes a polymer tape having a nozzle member formed therein and additionally having a first surface containing conductors thereon. The nozzle member includes a plurality of ink orifices. The print cartridge further includes a coverlayer having a first surface and a second surface. Each coverlayer surface has an adhesive coated thereon. The coverlayer first surface is bonded to the first surface of the polymer tape by the adhesive so as to substantially encapsulate the conductors and prevent electrical shorts caused by water vapor or ink. The adhesive includes a polymer of ethylene and glycidyl ethacrylate. Alternatively, the adhesive further includes an acrylic ester. The coverlayer is bonded to a print cartridge body containing an ink-retaining compartment therein, such that the nozzle member is in fluid communication with the ink-retaining compartment.
In yet another embodiment of the invention, a method of producing a print cartridge for use in an inkjet cartridge is provided. The method includes providing a polymer tape having a nozzle member formed therein and having a first surface containing conductors thereon. The nozzle member includes a plurality of ink orifices. A coverlayer having a first surface and a second surface is provided. Each coverlayer surface includes an adhesive coated thereon. The first surface of the coverlayer is bonded to the first surface of the polymer tape in order to substantially encapsulate the conductors and prevent electrical shorts caused by water vapor or ink. The adhesive includes a polymer of ethylene and glycidyl methacrylate. Alternatively, the adhesive further includes an acrylic ester. A print cartridge body containing an ink-retaining compartment therein is then provided. The second surface of the coverlayer is then bonded to the print cartridge body such that the nozzle member is in fluid communication with the ink-retaining compartment.
The particular structure of the printhead is not important in this patent application and various types of printheads, such as those with a nickel nozzle plate, may also be used with the present invention.